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Highly controllable and reproducible ZnO nanowire arrays growth with focused ion beam and low-temperature hydrothermal method
Applied Surface Science 317 (2014) 220–225
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Highly controllable and reproducible ZnO nanowire arrays growth with focused ion beam and low-temperature hydrothermal method Kaidi Diao a,b , Jicheng Zhang b , Minjie Zhou b , Yongjian Tang b , Shuxia Wang a , Xudong Cui b,∗ a b
Department of Applied Physics, Chongqing University, Chongqing, 400044, PR China Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, CAEP, Sichuan, 621900, China
a r t i c l e
i n f o
Article history: Received 11 April 2014 Received in revised form 18 July 2014 Accepted 15 August 2014 Available online 23 August 2014 Keywords: ZnO Nanowire arrays FIB Hydrothermal method ZnO nanodevices
a b s t r a c t In this work, high-quality ZnO nanowire arrays with controllable degrees over size, orientation, uniformity and periodicity are fabricated on GaN substrates with focused ion beam etching and lowtemperature hydrothermal method. Experimental results revealed that the patterned holes (i.e., shape, depth, size and period) have decisive impacts on the morphology of resulting arrays. Optimal conditions and ordered arrays are obtained in terms of functionality analysis for both patterned holes and hydrothermal method. A possible mechanism is proposed to interpret the growth process in and out of the pattern holes. Results show that this hybrid method exhibits good reproducibility for the fabrication of high-quality ZnO nanowire arrays with great potentials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Low dimensional ZnO nanostructures (e.g., nanobelts, nanowires, and nanorods etc.) could provide higher surface to volume ratio and electron mobility than their high dimensional components, exhibiting interesting electrical and optical properties [1–6]. They have found applications in the areas of nano-generators [1], sensors [2], solar cells [3], UV detectors [4], and field emission devices [5] etc. In order to obtain optimal performance, special requirements for ZnO nanowire arrays in terms of size, orientation, uniformity and periodicity are needed. Nowadays, challenges still remain to produce such well-controlled arrays and integrate with modern circuit techniques [1–6]. One main obstacle to produce customized ordered ZnO nanowire arrays is the patterning for the nanowire growth, in which the shape, size, depth of patterns are key to the nucleation and growth of wires in consequent processing steps. On the other hand, this special aspect has attracted less attention in published literatures. Therefore, to integrate the optimal fabrication process of ZnO nanowire arrays with modern technologies, understanding the roles of patterns and their working mechanisms is highly desired.
∗ Corresponding author. Tel./fax: +86 08162481486. E-mail addresses: [email protected], [email protected] (X. Cui). http://dx.doi.org/10.1016/j.apsusc.2014.08.088 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Several patterning techniques for nanostructures have been demonstrated [6–21]. Among these approaches, nanosphere lithography (NSL) [6,7] and nanoimprint lithography (NIL) [8,9] are two widely used approaches to obtain ordered metal catalyst dots for subsequent growth of nanowires. However, these two methods are sensitive to defects, distortions and domain boundaries that naturally appear when the colloids are crystallized on the substrate [6–9]. The resulting patterns generated with these techniques are very limited in terms of design and complexity [10,11]. Laser interference ablation (LIA) [12] and laser interference lithography (LIL) [13–16] are reliable to achieve period-adjustable patterns, but for smaller structures and for the subsequent growth control, they are still not good enough to combine with hydrothermal methods. For nanowire growth, a double exposure with a fixed rotation (usually 90◦ ) is needed. The pattern strongly depends on the accuracy of the rotation, which will affect the shape of the resulting exposed dots. In addition, for a good pattern, the whole process has to be optimized through a proper selection, including the laser wavelength, the resist, the exposition time, the variation of the incident angle of the laser and the laser irradiation dose. This makes the process complicated. In e-beam lithography (EBL) [17–20], the pattern depth is limited by the photoresist layer pre-coated on the substrate, and the electron beam cannot penetrate into substrate further and one must treat the substrate carefully to grow nanowires. Focused ion beam (FIB) is known as a powerful and reliable nanofabrication technique for patterns [21]. It has advantages
K. Diao et al. / Applied Surface Science 317 (2014) 220–225
over EBL of high current density, large energy density [22]. More importantly, high energetic ions in FIB could strongly interact with substrates (through etching and implanting), resulting in surface amorphization [23]. To better understand the nanowire growth on the substrate, especially to control the orientation of ZnO nanowire arrays on the same substrate, FIB is expected as a good option to investigate the influences of patterns on the final morphology of ZnO nanowire arrays in this work. To the best of our knowledge, there have few reports on the growth of ZnO nanowire arrays with this patterning approach. The low-temperature hydrothermal method has been widely used in the synthesis of 1D ZnO nanowires due to its low cost, flexibility, and possibility in-situ investigations and integration with general substrates [24]. For the wurtzite crystal structure, ZnO has both polar and non-polar faces. The main approach for achieving 1D growth is to explore the differences in surface chemistry between these two types of faces. Polar faces with surface dipoles are thermodynamically less stable than non-polar faces, usually suffering from rearrangement to minimize their surface energy and tend to grow more rapidly. By selectively promoting or suppressing growth on these facets, 1D growth could be achieved via the bottom up approach. In our experiments, hexamethylenetetramine (HMT) as a shape inducing molecule [25], with the concentration of 5 mM was used to promote the growth of ZnO nanowires along (0 0 0 1) direction. In this paper, by combining FIB with low-temperature hydrothermal method, we demonstrate an efficient two-step approach for the growth of high-quality ZnO nanowires arrays. With these two techniques, vertically aligned and periodically distributed individual ZnO nanowire arrays are obtained. The whole process is technologically controllable and reproducible. The investigations show that the method can be integrated with modern circuit techniques and has great potentials for sensor, piezoelectric devices, and optoelectronic devices fabrications.
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Fig. 1. 52◦ -tilted view of patterned hole fabricated by FIB, the depth is 400 nm.
nanowire arrays were investigated using a FEI field emission scanning electron microscopy (FESEM). A c-plane (0 0 0 1) GaN wafer with thickness of ∼5 m is selected as substrate for the growth of nanowires. GaN and ZnO have similar fundamental band-gap energy (3.37 eV for ZnO, 3.4 eV for GaN), the same wurtzite crystal symmetry, and a low mismatch of the lattice constant (1.8%) [26], these characteristics are favored in the experiments since no seed layer is needed when grow ZnO nanowires. It should be noted that under FIB etching, surface amorphization might be caused on the etched GaN surfaces by the bombardment of Ga+ , and thus, lattice mismatch would become larger. Based on our experimental method, we found that the surface amorphization effects on the growth can be neglected so we will not consider the issue in this work. 3. Results and discussions
2. Experimental 3.1. Effects of pattern depth All the chemicals were used as received without further purification. Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) and hexamethylenetetramine (HMT (CH2 )6 N4 ) were purchased from ChengDu Kelong Chemical Co., Ltd., China. Deionized water with a resistivity of 18.25 M cm−1 was obtained using up-water purification system at room temperature (Chengdu YouPu Technology Co. Ltd., China). GaN substrates were cleaned sequentially with acetone, ethanol and deionized water for 20 min, respectively, and then dried with nitrogen flow. Afterwards, the photoresist solution of polymethylmethacrylate (PMMA) was spin-coated on a GaN substrate to form a ∼200 nm thick PMMA layer. A FEI dual-beam FIB system (the accelerating voltage was set to 30 KeV in the experiment) was then used to produce the desired patterns on the PMMA coated GaN substrate. ZnO nanowire arrays were then fabricated using a low-temperature hydrothermal growth method, based on an aqueous solution having the same concentrations (5 mM) of Zn(NO3 )2 ·6H2 O and HMT (CH2 )6 N4 , as reported elsewhere [17].The substrates were mounted upside-down on a polytetrafluoroethylene sample holder to prevent any precipitates that formed in the nutrient solution from falling onto the substrates (which would have inhibited the growth of the nanowires). The growth time was 3 h, and the temperature was held at 95 ◦ C. After naturally cooling down to room temperature, the sample was taken out of the solution, and the PMMA photoresist layer was dissolved by putting the substrate into acetone for 1 min. Then the sample was thoroughly rinsed with deionized water and dried at 60 ◦ C in air. The resulting ZnO
First, a bitmap file was utilized to generate the desired growth patterns with FIB. To balance the total etching-time and the feasible accuracy of focused beam, the probe current is fixed to 48 pA in the experiment. Different hole depths (from ∼150 nm to ∼400 nm) are fabricated, the hole diameter is set to be ∼410 nm, and the period is 2 m to investigate the effects of pattern depth on the growth of nanowires. Arrays are then growing with low-temperature hydrothermal process under same conditions for comparison reasons. Fig. 1 shows the patterned hole with FIB. The appearance is relatively complex: the upper part is a column, while the bottom is a cone [27]. In this case, we can see that FIB patterning involves with two effects: etch and implantation [22,23]. Besides etching functionality, ions (Ga+ is used in our work) may also penetrate into the GaN substrate up to several dozens of nanometers, depending on the used ions’ energy (500 eV–30 KeV). The curved surface, together with the modified GaN substrate will induce the nucleation and growth of ZnO nanowire (we will investigate the implantation effect elsewhere). In the following, we then investigate the effects of pattern depths on the final arrays under same hydrothermal growth conditions. The grown arrays are shown in Fig. 2. When the hole depth is ∼150 nm (Fig. 2a, less than the thickness of PMMA photoresist layer), no ZnO nanowire grows out of the holes,. This is because that ZnO could not form nucleation on the organic PMMA surface. As the hole depth increases to ∼200 nm, ZnO nanowires grow out of the holes (Fig. 2b), with relatively large width distribution, ranging
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Fig. 2. SEM images of ZnO nanowire arrays obtained at different hole depths: (a) ∼150 nm, (b) ∼200 nm, (c) ∼300 nm, (d) ∼400 nm. The scale bars correspond to 3 m.
from ∼100 nm to ∼1.2 m. Note that most nanowires are randomly oriented, and only several ones kept vertically. In addition, we observed that several nanowires grew out of the individual hole (Fig. 2b). Continuing increasing the hole depth to 300 nm (larger than the thickness of PMMA photoresist layer), we can see that more ZnO nanowires vertically aligned on the substrate and only several ones grew with poor orientations (Fig. 2c). For most holes, one hole is with only one wire. Nevertheless, the ZnO nanowires still had large width distribution (Fig. 2c). Further increasing the hole depth to ∼400 nm (Fig. 2d), almost vertically aligned ZnO nanowire arrays are obtained with good orientations. Note that several ZnO nanowires grew from one individual hole merged together to form a big one (Fig. 2d). Thus, cares must be taken when use this hybrid method. In order to understand the impact of hole depth on the morphology of ZnO nanowire arrays, we carefully investigate the underlying mechanisms. We assume that, at the beginning, the nucleation and growth of ZnO nanowire could occur randomly on the curve surface of cone shaped GaN holes (Fig. 3a and b). In addition, due to surface amorphization caused by the bombardment of Ga+ , and the cone shape of growth hole (Fig. 1, Fig. 3a and b), we also assumed ZnO nanowire grew in the random direction of curve surface. Thus, when the growth hole is shallow (∼200 nm, Fig. 3a), not all ZnO nanowires in different sites growing on the curve surface had influence on each other. Some nanowires gradually grew out of holes along the initial growth direction due to the absence of enough physical confinement from growth holes. The others partly touched and merged together to form wider ones. In this case, several ZnO nanowires with different growth directions and large width distribution would grow in a single hole, as shown in Fig. 4a. While increasing the hole depth (Fig. 3b, Fig. 4a–c), due to enough
physical confinement of holes, ZnO nanowires contacted with each other before growing out of holes. Then, they all merged together, and finally formed a high-quality ZnO nanowire vertically aligned on the substrate, as shown in Fig. 3b and Fig. 4d. Therefore, deeper holes are advantageous to grow high quality nanowires as shown in the SEM images with different hole depth. We have to mention here that the hole depth and diameter also will influence the diameter of resulting wires. Thus, optimization is needed in this step and one should take this and the consequent hydrothermal growth
Fig. 3. Schematic illustrating the growth processes of ZnO nanowires under different hole depths: (a) shallow hole; (b) deep hole.
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Fig. 4. SEM images of ZnO nanowires obtained under the different hole depths: (a)–(c) ∼200 nm, (d) ∼400 nm.
into consideration. From Fig. 3 and Fig. 4, we can see that the proposed mechanism agrees very well with our analysis. In following work, we then chose the hole depth 400 nm as the optimal value to fabricate our ZnO nanowire arrays. 3.2. Effects of pattern diameter and period Besides the hole depth, the effects of pattern diameter and period must be also taken into considerations, since the designed optical properties are greatly impacted by the diameter and period [1–6]. Fig. 5a–c showed the typical SEM images of two-dimensional periodic growth hole patterns with different hole sizes from ∼410 nm to ∼140 nm, and different hole periods from 2 m to 0.5 m. The hole depth is fixed to 400 nm. After low-temperature hydrothermal growth, high-quality ZnO nanowire arrays with different width and period were obtained (Fig. 5d–f). The average length of the nanowires for the three types was the same (∼5 m), while the average diameter were ∼1200, 600 and 400 nm for the 410, 200and 140 nm hole size, respectively. The period for each type of nanowire array was consistent with that of the hole array. As previously reported, the length and diameter of ZnO nanowires could
be controlled mainly by the concentration of nutrient solution, growth time and temperature [28]. However, in our case, the hole size was also a key factor to control the width of ZnO nanowires. As shown in Fig. 5f, several nanowires were merged together during the growth processes because of the space of neighbor growth holes was small. In our experiment, increasing the distance of the neighbor holes could eliminate the mergence. Besides, we should note that in our procedure the diameter of nanowires for the three types of hole diameters was about three times of the corresponding diameter of holes. This relation can be used to control the diameter of grown nanowires and meantime shows the limitation of this hybrid method. 3.3. Hydrothermal method The choices of complex ligand and Zn counter-ion effects are important factors in a hydrothermal procedure [24,29]. Several key points must be noticed in this procedure: (1) It is known that weak bases such as amines hydrolyze in aqueous Zn environments and slowly release hydroxide ions, forming various Zn-hydroxide complexes [24]. When ethylenediamine (EN) was used as the weak
Fig. 5. SEM images of two-dimensional periodic growth hole patterns with different sizes and periods: (a) ∼410 nm, 2 m, (b) ∼200 nm, 1 m, (c) ∼140 nm, 0.5 m, and ZnO nanowire arrays: (d), (e) and (f) corresponding to (a), (b) and (c), respectively. All the scale bars are 2 m.
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Fig. 6. The grown nanowire arrays under the same conditions of solution concentration, grown time and temperature (a) with surfactant, the average diameter of nanowire is ∼90 nm; (b) without surfactant, the average diameter of nanowire is ∼1 m.
base, at low zinc concentration and high hydroxide concentration, regardless of the Zn counter-ions, wire-shape morphology will be created. When triethanolamine (TEA) was used, only particles but no wires will be obtained. When HMT was used, high-quality ZnO nanowires will be obtained in general [24,29]. Although HMT has been commonly used to synthesize 1D ZnO nanostructures [28,30], its detailed role is still of argument in literatures. It is generally accepted that HMT acts as a pH buffer and will slowly, steadily releases hydroxide ions [31]. The hydroxide ions further react with Zn2+ to form zinc complexes. Nevertheless, Sugunan et al. [25] believed that HMT acted as a shape inducing molecule by preferentially attaching to the non-polar facets of ZnO nanowires, thereby exposing only the (0 0 0 1) plane for epitaxial growth. Thus a preferential growth along the [0 0 0 1] direction is possible to be achieved. (2) The control of the supersaturation of reactants. High supersaturation levels favor for nucleation, while low supersaturation levels favor for crystal growth. It is believed that low supersaturation levels of reactants will favor 1D nanomaterial’s growth. In order to maintain low supersaturation levels during the growth process, the solution concentration was set as low as 5 mM. Therefore, in our experiment, the growth processes could be separated into three steps during the formation of ZnO nanowire arrays. First, in the deep holes, due to the space limitation, several ZnO nanowires in a hole gradually merged together to form a big one. The second step was the merged ZnO nanowires growth in the growth holes. In this stage, the ZnO nanowires grew epitaxially with c-axis orientation in the precursor solution. Due to the physical confinement of the growth holes, the ZnO nanowires growing out of the holes suffered from the same lateral dimension with the patterned holes. As the growth proceed, there was no lateral confinement for it, and the nanowires could grow both vertically and laterally, but apparently it is faster in the vertical direction than that of in the lateral direction, caused by the differences in surface energy ¯ > ZnO(1010)). ¯ for different crystal planes (ZnO(0001) > ZnO(1120) Even though the nanowires would expand laterally when they grew out of the confining holes, the developed nanowires showed proportional sizes to the holes accordingly. Therefore, we could still tune the diameter of the nanowires in a very responsive and reliable fashion according to the proportional relationship between the nanowire diameter and the hole size. Note that in the growth process, the surfactant plays a dominant role in the aspect of suppressing the lateral growth through preferentially adsorbed on the nonpolar crystal planes resulting in a strong capping effect. Fig. 6 shows the grown nanowire arrays under the same conditions of solution concentration, grown time and temperature with and without surfactant. The one with surfactant is well-controlled along its lateral direction than that of without surfactant, the diameter of nanowires is much smaller.
4. Conclusion In conclusion, we have demonstrated an efficient two-step approach to fabricate high-quality ZnO nanowires arrays with a high degree of control over size, orientation and periodicity, through the combination of FIB and low-temperature hydrothermal growth process. The roles of patterns and underlying mechanisms are carefully investigated and discussed. The significance of this study is twofold. Firstly, FIB has the advantages of EBL over other methods in the fabrication of periodic patterns, but more importantly, it could heavily interact with GaN substrate to change the microstructure of substrate surface, so that it facilitates to investigate the growth process of ZnO nanowire. Secondly, by using the low-temperature hydrothermal method, the catalyst could be eliminated so that the integration with silicon based technology is possible. The morphology of ZnO nanowire arrays was found to be strongly dependent on the growth holes. Understanding the growth hole parameters that affect the final morphology and the growth mechanism was important for potential applications in ZnO nanowire array-based nanodevices.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Highly controllable and reproducible ZnO nanowire arrays growth with focused ion beam and low-temperature hydrothermal method Kaidi Diao a,b , Jicheng Zhang b , Minjie Zhou b , Yongjian Tang b , Shuxia Wang a , Xudong Cui b,∗ a b
Department of Applied Physics, Chongqing University, Chongqing, 400044, PR China Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, CAEP, Sichuan, 621900, China
a r t i c l e
i n f o
Article history: Received 11 April 2014 Received in revised form 18 July 2014 Accepted 15 August 2014 Available online 23 August 2014 Keywords: ZnO Nanowire arrays FIB Hydrothermal method ZnO nanodevices
a b s t r a c t In this work, high-quality ZnO nanowire arrays with controllable degrees over size, orientation, uniformity and periodicity are fabricated on GaN substrates with focused ion beam etching and lowtemperature hydrothermal method. Experimental results revealed that the patterned holes (i.e., shape, depth, size and period) have decisive impacts on the morphology of resulting arrays. Optimal conditions and ordered arrays are obtained in terms of functionality analysis for both patterned holes and hydrothermal method. A possible mechanism is proposed to interpret the growth process in and out of the pattern holes. Results show that this hybrid method exhibits good reproducibility for the fabrication of high-quality ZnO nanowire arrays with great potentials. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Low dimensional ZnO nanostructures (e.g., nanobelts, nanowires, and nanorods etc.) could provide higher surface to volume ratio and electron mobility than their high dimensional components, exhibiting interesting electrical and optical properties [1–6]. They have found applications in the areas of nano-generators [1], sensors [2], solar cells [3], UV detectors [4], and field emission devices [5] etc. In order to obtain optimal performance, special requirements for ZnO nanowire arrays in terms of size, orientation, uniformity and periodicity are needed. Nowadays, challenges still remain to produce such well-controlled arrays and integrate with modern circuit techniques [1–6]. One main obstacle to produce customized ordered ZnO nanowire arrays is the patterning for the nanowire growth, in which the shape, size, depth of patterns are key to the nucleation and growth of wires in consequent processing steps. On the other hand, this special aspect has attracted less attention in published literatures. Therefore, to integrate the optimal fabrication process of ZnO nanowire arrays with modern technologies, understanding the roles of patterns and their working mechanisms is highly desired.
∗ Corresponding author. Tel./fax: +86 08162481486. E-mail addresses: [email protected], [email protected] (X. Cui). http://dx.doi.org/10.1016/j.apsusc.2014.08.088 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Several patterning techniques for nanostructures have been demonstrated [6–21]. Among these approaches, nanosphere lithography (NSL) [6,7] and nanoimprint lithography (NIL) [8,9] are two widely used approaches to obtain ordered metal catalyst dots for subsequent growth of nanowires. However, these two methods are sensitive to defects, distortions and domain boundaries that naturally appear when the colloids are crystallized on the substrate [6–9]. The resulting patterns generated with these techniques are very limited in terms of design and complexity [10,11]. Laser interference ablation (LIA) [12] and laser interference lithography (LIL) [13–16] are reliable to achieve period-adjustable patterns, but for smaller structures and for the subsequent growth control, they are still not good enough to combine with hydrothermal methods. For nanowire growth, a double exposure with a fixed rotation (usually 90◦ ) is needed. The pattern strongly depends on the accuracy of the rotation, which will affect the shape of the resulting exposed dots. In addition, for a good pattern, the whole process has to be optimized through a proper selection, including the laser wavelength, the resist, the exposition time, the variation of the incident angle of the laser and the laser irradiation dose. This makes the process complicated. In e-beam lithography (EBL) [17–20], the pattern depth is limited by the photoresist layer pre-coated on the substrate, and the electron beam cannot penetrate into substrate further and one must treat the substrate carefully to grow nanowires. Focused ion beam (FIB) is known as a powerful and reliable nanofabrication technique for patterns [21]. It has advantages
K. Diao et al. / Applied Surface Science 317 (2014) 220–225
over EBL of high current density, large energy density [22]. More importantly, high energetic ions in FIB could strongly interact with substrates (through etching and implanting), resulting in surface amorphization [23]. To better understand the nanowire growth on the substrate, especially to control the orientation of ZnO nanowire arrays on the same substrate, FIB is expected as a good option to investigate the influences of patterns on the final morphology of ZnO nanowire arrays in this work. To the best of our knowledge, there have few reports on the growth of ZnO nanowire arrays with this patterning approach. The low-temperature hydrothermal method has been widely used in the synthesis of 1D ZnO nanowires due to its low cost, flexibility, and possibility in-situ investigations and integration with general substrates [24]. For the wurtzite crystal structure, ZnO has both polar and non-polar faces. The main approach for achieving 1D growth is to explore the differences in surface chemistry between these two types of faces. Polar faces with surface dipoles are thermodynamically less stable than non-polar faces, usually suffering from rearrangement to minimize their surface energy and tend to grow more rapidly. By selectively promoting or suppressing growth on these facets, 1D growth could be achieved via the bottom up approach. In our experiments, hexamethylenetetramine (HMT) as a shape inducing molecule [25], with the concentration of 5 mM was used to promote the growth of ZnO nanowires along (0 0 0 1) direction. In this paper, by combining FIB with low-temperature hydrothermal method, we demonstrate an efficient two-step approach for the growth of high-quality ZnO nanowires arrays. With these two techniques, vertically aligned and periodically distributed individual ZnO nanowire arrays are obtained. The whole process is technologically controllable and reproducible. The investigations show that the method can be integrated with modern circuit techniques and has great potentials for sensor, piezoelectric devices, and optoelectronic devices fabrications.
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Fig. 1. 52◦ -tilted view of patterned hole fabricated by FIB, the depth is 400 nm.
nanowire arrays were investigated using a FEI field emission scanning electron microscopy (FESEM). A c-plane (0 0 0 1) GaN wafer with thickness of ∼5 m is selected as substrate for the growth of nanowires. GaN and ZnO have similar fundamental band-gap energy (3.37 eV for ZnO, 3.4 eV for GaN), the same wurtzite crystal symmetry, and a low mismatch of the lattice constant (1.8%) [26], these characteristics are favored in the experiments since no seed layer is needed when grow ZnO nanowires. It should be noted that under FIB etching, surface amorphization might be caused on the etched GaN surfaces by the bombardment of Ga+ , and thus, lattice mismatch would become larger. Based on our experimental method, we found that the surface amorphization effects on the growth can be neglected so we will not consider the issue in this work. 3. Results and discussions
2. Experimental 3.1. Effects of pattern depth All the chemicals were used as received without further purification. Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O) and hexamethylenetetramine (HMT (CH2 )6 N4 ) were purchased from ChengDu Kelong Chemical Co., Ltd., China. Deionized water with a resistivity of 18.25 M cm−1 was obtained using up-water purification system at room temperature (Chengdu YouPu Technology Co. Ltd., China). GaN substrates were cleaned sequentially with acetone, ethanol and deionized water for 20 min, respectively, and then dried with nitrogen flow. Afterwards, the photoresist solution of polymethylmethacrylate (PMMA) was spin-coated on a GaN substrate to form a ∼200 nm thick PMMA layer. A FEI dual-beam FIB system (the accelerating voltage was set to 30 KeV in the experiment) was then used to produce the desired patterns on the PMMA coated GaN substrate. ZnO nanowire arrays were then fabricated using a low-temperature hydrothermal growth method, based on an aqueous solution having the same concentrations (5 mM) of Zn(NO3 )2 ·6H2 O and HMT (CH2 )6 N4 , as reported elsewhere [17].The substrates were mounted upside-down on a polytetrafluoroethylene sample holder to prevent any precipitates that formed in the nutrient solution from falling onto the substrates (which would have inhibited the growth of the nanowires). The growth time was 3 h, and the temperature was held at 95 ◦ C. After naturally cooling down to room temperature, the sample was taken out of the solution, and the PMMA photoresist layer was dissolved by putting the substrate into acetone for 1 min. Then the sample was thoroughly rinsed with deionized water and dried at 60 ◦ C in air. The resulting ZnO
First, a bitmap file was utilized to generate the desired growth patterns with FIB. To balance the total etching-time and the feasible accuracy of focused beam, the probe current is fixed to 48 pA in the experiment. Different hole depths (from ∼150 nm to ∼400 nm) are fabricated, the hole diameter is set to be ∼410 nm, and the period is 2 m to investigate the effects of pattern depth on the growth of nanowires. Arrays are then growing with low-temperature hydrothermal process under same conditions for comparison reasons. Fig. 1 shows the patterned hole with FIB. The appearance is relatively complex: the upper part is a column, while the bottom is a cone [27]. In this case, we can see that FIB patterning involves with two effects: etch and implantation [22,23]. Besides etching functionality, ions (Ga+ is used in our work) may also penetrate into the GaN substrate up to several dozens of nanometers, depending on the used ions’ energy (500 eV–30 KeV). The curved surface, together with the modified GaN substrate will induce the nucleation and growth of ZnO nanowire (we will investigate the implantation effect elsewhere). In the following, we then investigate the effects of pattern depths on the final arrays under same hydrothermal growth conditions. The grown arrays are shown in Fig. 2. When the hole depth is ∼150 nm (Fig. 2a, less than the thickness of PMMA photoresist layer), no ZnO nanowire grows out of the holes,. This is because that ZnO could not form nucleation on the organic PMMA surface. As the hole depth increases to ∼200 nm, ZnO nanowires grow out of the holes (Fig. 2b), with relatively large width distribution, ranging
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Fig. 2. SEM images of ZnO nanowire arrays obtained at different hole depths: (a) ∼150 nm, (b) ∼200 nm, (c) ∼300 nm, (d) ∼400 nm. The scale bars correspond to 3 m.
from ∼100 nm to ∼1.2 m. Note that most nanowires are randomly oriented, and only several ones kept vertically. In addition, we observed that several nanowires grew out of the individual hole (Fig. 2b). Continuing increasing the hole depth to 300 nm (larger than the thickness of PMMA photoresist layer), we can see that more ZnO nanowires vertically aligned on the substrate and only several ones grew with poor orientations (Fig. 2c). For most holes, one hole is with only one wire. Nevertheless, the ZnO nanowires still had large width distribution (Fig. 2c). Further increasing the hole depth to ∼400 nm (Fig. 2d), almost vertically aligned ZnO nanowire arrays are obtained with good orientations. Note that several ZnO nanowires grew from one individual hole merged together to form a big one (Fig. 2d). Thus, cares must be taken when use this hybrid method. In order to understand the impact of hole depth on the morphology of ZnO nanowire arrays, we carefully investigate the underlying mechanisms. We assume that, at the beginning, the nucleation and growth of ZnO nanowire could occur randomly on the curve surface of cone shaped GaN holes (Fig. 3a and b). In addition, due to surface amorphization caused by the bombardment of Ga+ , and the cone shape of growth hole (Fig. 1, Fig. 3a and b), we also assumed ZnO nanowire grew in the random direction of curve surface. Thus, when the growth hole is shallow (∼200 nm, Fig. 3a), not all ZnO nanowires in different sites growing on the curve surface had influence on each other. Some nanowires gradually grew out of holes along the initial growth direction due to the absence of enough physical confinement from growth holes. The others partly touched and merged together to form wider ones. In this case, several ZnO nanowires with different growth directions and large width distribution would grow in a single hole, as shown in Fig. 4a. While increasing the hole depth (Fig. 3b, Fig. 4a–c), due to enough
physical confinement of holes, ZnO nanowires contacted with each other before growing out of holes. Then, they all merged together, and finally formed a high-quality ZnO nanowire vertically aligned on the substrate, as shown in Fig. 3b and Fig. 4d. Therefore, deeper holes are advantageous to grow high quality nanowires as shown in the SEM images with different hole depth. We have to mention here that the hole depth and diameter also will influence the diameter of resulting wires. Thus, optimization is needed in this step and one should take this and the consequent hydrothermal growth
Fig. 3. Schematic illustrating the growth processes of ZnO nanowires under different hole depths: (a) shallow hole; (b) deep hole.
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Fig. 4. SEM images of ZnO nanowires obtained under the different hole depths: (a)–(c) ∼200 nm, (d) ∼400 nm.
into consideration. From Fig. 3 and Fig. 4, we can see that the proposed mechanism agrees very well with our analysis. In following work, we then chose the hole depth 400 nm as the optimal value to fabricate our ZnO nanowire arrays. 3.2. Effects of pattern diameter and period Besides the hole depth, the effects of pattern diameter and period must be also taken into considerations, since the designed optical properties are greatly impacted by the diameter and period [1–6]. Fig. 5a–c showed the typical SEM images of two-dimensional periodic growth hole patterns with different hole sizes from ∼410 nm to ∼140 nm, and different hole periods from 2 m to 0.5 m. The hole depth is fixed to 400 nm. After low-temperature hydrothermal growth, high-quality ZnO nanowire arrays with different width and period were obtained (Fig. 5d–f). The average length of the nanowires for the three types was the same (∼5 m), while the average diameter were ∼1200, 600 and 400 nm for the 410, 200and 140 nm hole size, respectively. The period for each type of nanowire array was consistent with that of the hole array. As previously reported, the length and diameter of ZnO nanowires could
be controlled mainly by the concentration of nutrient solution, growth time and temperature [28]. However, in our case, the hole size was also a key factor to control the width of ZnO nanowires. As shown in Fig. 5f, several nanowires were merged together during the growth processes because of the space of neighbor growth holes was small. In our experiment, increasing the distance of the neighbor holes could eliminate the mergence. Besides, we should note that in our procedure the diameter of nanowires for the three types of hole diameters was about three times of the corresponding diameter of holes. This relation can be used to control the diameter of grown nanowires and meantime shows the limitation of this hybrid method. 3.3. Hydrothermal method The choices of complex ligand and Zn counter-ion effects are important factors in a hydrothermal procedure [24,29]. Several key points must be noticed in this procedure: (1) It is known that weak bases such as amines hydrolyze in aqueous Zn environments and slowly release hydroxide ions, forming various Zn-hydroxide complexes [24]. When ethylenediamine (EN) was used as the weak
Fig. 5. SEM images of two-dimensional periodic growth hole patterns with different sizes and periods: (a) ∼410 nm, 2 m, (b) ∼200 nm, 1 m, (c) ∼140 nm, 0.5 m, and ZnO nanowire arrays: (d), (e) and (f) corresponding to (a), (b) and (c), respectively. All the scale bars are 2 m.
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Fig. 6. The grown nanowire arrays under the same conditions of solution concentration, grown time and temperature (a) with surfactant, the average diameter of nanowire is ∼90 nm; (b) without surfactant, the average diameter of nanowire is ∼1 m.
base, at low zinc concentration and high hydroxide concentration, regardless of the Zn counter-ions, wire-shape morphology will be created. When triethanolamine (TEA) was used, only particles but no wires will be obtained. When HMT was used, high-quality ZnO nanowires will be obtained in general [24,29]. Although HMT has been commonly used to synthesize 1D ZnO nanostructures [28,30], its detailed role is still of argument in literatures. It is generally accepted that HMT acts as a pH buffer and will slowly, steadily releases hydroxide ions [31]. The hydroxide ions further react with Zn2+ to form zinc complexes. Nevertheless, Sugunan et al. [25] believed that HMT acted as a shape inducing molecule by preferentially attaching to the non-polar facets of ZnO nanowires, thereby exposing only the (0 0 0 1) plane for epitaxial growth. Thus a preferential growth along the [0 0 0 1] direction is possible to be achieved. (2) The control of the supersaturation of reactants. High supersaturation levels favor for nucleation, while low supersaturation levels favor for crystal growth. It is believed that low supersaturation levels of reactants will favor 1D nanomaterial’s growth. In order to maintain low supersaturation levels during the growth process, the solution concentration was set as low as 5 mM. Therefore, in our experiment, the growth processes could be separated into three steps during the formation of ZnO nanowire arrays. First, in the deep holes, due to the space limitation, several ZnO nanowires in a hole gradually merged together to form a big one. The second step was the merged ZnO nanowires growth in the growth holes. In this stage, the ZnO nanowires grew epitaxially with c-axis orientation in the precursor solution. Due to the physical confinement of the growth holes, the ZnO nanowires growing out of the holes suffered from the same lateral dimension with the patterned holes. As the growth proceed, there was no lateral confinement for it, and the nanowires could grow both vertically and laterally, but apparently it is faster in the vertical direction than that of in the lateral direction, caused by the differences in surface energy ¯ > ZnO(1010)). ¯ for different crystal planes (ZnO(0001) > ZnO(1120) Even though the nanowires would expand laterally when they grew out of the confining holes, the developed nanowires showed proportional sizes to the holes accordingly. Therefore, we could still tune the diameter of the nanowires in a very responsive and reliable fashion according to the proportional relationship between the nanowire diameter and the hole size. Note that in the growth process, the surfactant plays a dominant role in the aspect of suppressing the lateral growth through preferentially adsorbed on the nonpolar crystal planes resulting in a strong capping effect. Fig. 6 shows the grown nanowire arrays under the same conditions of solution concentration, grown time and temperature with and without surfactant. The one with surfactant is well-controlled along its lateral direction than that of without surfactant, the diameter of nanowires is much smaller.
4. Conclusion In conclusion, we have demonstrated an efficient two-step approach to fabricate high-quality ZnO nanowires arrays with a high degree of control over size, orientation and periodicity, through the combination of FIB and low-temperature hydrothermal growth process. The roles of patterns and underlying mechanisms are carefully investigated and discussed. The significance of this study is twofold. Firstly, FIB has the advantages of EBL over other methods in the fabrication of periodic patterns, but more importantly, it could heavily interact with GaN substrate to change the microstructure of substrate surface, so that it facilitates to investigate the growth process of ZnO nanowire. Secondly, by using the low-temperature hydrothermal method, the catalyst could be eliminated so that the integration with silicon based technology is possible. The morphology of ZnO nanowire arrays was found to be strongly dependent on the growth holes. Understanding the growth hole parameters that affect the final morphology and the growth mechanism was important for potential applications in ZnO nanowire array-based nanodevices.
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